The present invention relates to an arrangement for stretching thermoplastic fibers of synthetic polymers, especially polypropylene, polyester or polyamide, for producing high strength yarns.
It is known that for obtaining the desired mechanical properties of synthetic fibers, a stretching is required. The optimal stretching must be performed with as high as possible stretching ratio in a narrowly limited temperature interval. By the stretching, energy is supplied to the fibers. The magnitude of the energy depends on the stretching ratio and the stretching force. The fibers are heated due to the supplied energy. The stretching ratio must be limited to a value at which the heating produces no temperature increase which can lead to a reduction of the strength of the fibers and thereby to fiber breakage.
For providing higher stretching ratios, stretching arrangements have been developed in which the fibers passes several successive stretching zones. In the stretching zone which is adjacent to the stretching mechanism the stretching force acting in the fiber reaches its maximal value. In direction to the delivery mechanism, or in other words in direction which is opposite to the fiber running direction, the stretching force reduces in a stepped manner. Thus, the stretching is performed, with the exception of the last stretching zone, with reducing stretching force. Thereby with the same stretching ratio, less energy is introduced. This means that the higher stretching ratios are possible without tearing off the fibers.
In the German document DE-AS No. 1,950,743 additional rollers are arranged between the delivery mechanism and the stretching mechanism. They act as separating members between the successive stretching zones. Each roller is provided with a drive and has a peripheral speed at which a slippage is produced between the roller and the fiber. When the peripheral speed of the roller is slower than the fiber speed at the contact point or when it is opposite to the fiber speed, the friction force acts between the roller and the fiber so as to reduce the stretching force. In other words, the stretching force upstream of the roller is smaller than downstream of the same. The value of the stretching force depends on the relative speed between the fiber and the roller and on the surface property of the roller. The roller can serve as a heating or cooling element as well. In this known arrangement it is very difficult to adjust the desired sliding friction force and to maintain it permanently during the operation. A disadvantage of this arrangement is that during the sliding friction the fiber is loaded mechanically. A further disadvantage is the unavoidable friction heat which leads to a temperature increase and to reduction of the strength of the fiber.
The German document DE-OS No. 3,540,181 discloses a stretching arrangement in a heated water bath with three axes-parallel deviating bars which preferably are not rotatable. The fibers surround the deviating bars at alternating sides in a zig-zag shape. Due to the grouped support of the bars, the number of the effective deviating bars and the angle of wrap can be changed. In this manner, the number of the stretching zones and the stepping of the stretching force is varied. In this arrangement also it is difficult to maintain the sliding friction force permanent in the operation. A disadvantage of this arrangement is also that the available temperature region is limited here by the dew point of water.
U.S. Pat. No. 3,978,192 describes a stretching arrangement with three rotatable rotation bodies arranged at short distances near one another and formed as deviating elements. They also can be heated. The axes of the rotation elements can be slightly inclined relative to one another. At least one of both rotation element is conical while the increase of its radius is performed in the axial direction in form of small steps. The second rotation element can also be conical, and also it can be composed of a series of rollers which are located near one another and loosely sit on an axle. The fiber surrounds both rotation elements in several helical-like convolutions and moves in direction of the increased radius of the cone. The fiber lies without slippage on the conical surface and is stretched at each revolution to a predetermined value corresponding to the increase of the radius. Therefore, a uniform stepped stretching without the sliding friction is insured. What is not insured, however, is an optimal stepping of the stretching force.
The Swiss Pat. No. 284,352 describes a stretching arrangement with at least one deviating element located between the delivery mechanism and the stretching mechanism and formed for example as a heated roller so as to deviate the fiber by at least 90.degree. from the straight line. Due to the deviation the structure of the treatment product is loosened so that the flow takes place immediately afterwards. In this manner it must be ensured that the stretching is performed always at the same place, namely on the deviating element. A subdivision of the stretching process into several stretching zones with different stretching force is not provided and cannot provided in this arrangement.